Apparatus for corneal surgery

ABSTRACT

An apparatus for corneal surgery to correct a refractive error by ablating corneal tissue with a laser beam emitted from a laser source and delivered onto a cornea of a patient&#39;s eye with a light delivering optical system, the apparatus comprising an irradiation area limiting device for limiting an irradiation area of the laser beam and for varying the irradiation area, a first control device for controlling the irradiation area limiting device so as to reduce an ablation amount as the laser beam irradiates further away from a flattest meridian of astigmatism whereby effecting astigmatic correction, a second control device for controlling the irradiation area limiting device so as to increase an ablation amount as the laser beam irradiates further away from a steepest meridian of astigmatism whereby effecting astigmatic correction, and an arithmetic device for dividing a refractive power required for astigmatic correction into halves approximately-equally so that an approximately half of the astigmatic correction is achieved by the first control device and the residual astigmatic correction is achieved by the second control device.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an apparatus for corneal surgeryto correct a refractive error by ablating corneal tissue with a laserbeam, and more particularly to an apparatus suitable for astigmaticcorrection.

[0003] 2. Description of Related Art

[0004] An apparatus for corneal surgery to correct an refractive errorof an eyeball by ablating a corneal surface with an excimer laser isconventionally known.

[0005] However, astigmatic correction, especially myopic astigmaticcorrection, performed by using this kind of apparatus has a problem thatis a hyperopic shift of spherical component often occurs after such acorrection. To address this problem, two methods have been suggested:one is to minimize a size of ablation area in a meridian direction forcorrection so as to reduce affect of the hyperopic shift, and the otheris to combine myopic astigmatic correction and hyperopic astigmaticcorrection in order to correct myopic astigmatism.

[0006] The latter method is to estimate a hyperopic shift of thespherical component which will be developed after the correction of themyopic astigmatism so as to combine ablation of each correction inconsideration of the estimated hyperopic shift. For example, in the casewhere a hyperopic shift of 33% estimated, upon correcting simple myopicastigmatism of which correction amount is S=0 D, C=−3.0 D and A=0°, 75%of the correction amount, that is C=−2.25 D and A=0°, is removed byablation for myopic astigmatic correction. As the result, the hyperopicshift of 0.75 D, which accounts for 33%, occurs and the cornealrefractive power after the ablation will be S=+0.75 D, C=−0.75 D andA=0°. Here, replacing the sign of the astigmatism with a plus sign, theresidual correction amount will be S=0 D, C=+0.75 D and A=0°, which isequal to 25% of the correction amount to be corrected by ablation forhyperopic astigmatic correction.

[0007] Further, in the case of mixed astigmatism of which sphericalequivalent is 0, 37% of the correction amount is achieved by myopicastigmatic correction and 63% is achieved by hyperopic astigmaticcorrection. For example, in the case where the correction amount isS=+2.0 D, C=−4.0 D and A=0° by performing correction for S=0 D, C=−1.5 Dand A=0° which is equal to 37% of the correction amount, C=−4.0D in thiscase, a hyperopic shift of 0.5 D which accounts for 33% occurs and theresulting refractive power will be S=+2.5 D, C =−2.5 D and A =0°.Replacing the sign of the astigmatism with a plus sign, the residualcorrection amount will be S=0 D, C=+2.5 D and A=90°, which is equal to63% of the correction amount to be corrected by hyperopic astigmaticcorrection.

[0008] In the former of the above methods, since the range (the size)where the correction is effected is narrow, there is a problem that thepatient may experience halos or glare when, for example, the pupil hasdilated at night.

[0009] The latter method has been suggested to address the problem arisein the former method. Yet, there is another problem that the hyper shiftrate needs to be obtained as an empirical value to estimate hyper shiftof the spherical component caused by myopic astigmatic correction. Inaddition, this method has been applied only for correction of a certainlimited range.

SUMMARY OF THE INVENTION

[0010] The present invention has been made in view of the abovecircumstances and has an object to overcome the above problems and toprovide an apparatus for corneal surgery which eliminates the need toobtain the hyper shift rate caused by astigmatic correction as anempirical value, and which can reduce adverse effect of change in thespherical component without a specific limitation on a corrective rangeupon astigmatic correction.

[0011] Additional objects and advantages of the invention will be setforth in part in the description which follows and in part will beobvious from the description, or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

[0012] To achieve the objects and in accordance with the purpose of thepresent invention, as embodied and broadly described herein, anapparatus for corneal surgery to correct a refractive error by ablatingcorneal tissue with a laser beam emitted from a laser source anddelivered onto a cornea of a patient's eye with a light deliveringoptical system, the apparatus comprises irradiation area limiting meansfor limiting an irradiation area of the laser beam and for varying theirradiation area, first control means for controlling the irradiationarea limiting means so as to reduce an ablation amount as the laser beamirradiates further away from a flattest meridian of astigmatism wherebyeffecting astigmatic correction, second control means for controllingthe irradiation area limiting means so as to increase an ablation amountas the laser beam irradiates further away from a steepest meridian ofastigmatism whereby effecting astigmatic correction, and arithmeticmeans for dividing a refractive power required for astigmatic correctioninto halves approximately equally so that an approximately half of theastigmatic correction is achieved by the first control means and theresidual astigmatic correction is achieved by the second control means.

[0013] In another aspect of the present invention, an apparatus forcorneal surgery to correct a refractive error by ablating an opticalzone of a cornea with a laser beam, the apparatus comprises input meansutilized for inputting each data necessary for correction, an opticalsystem, first control means for controlling the optical system so as tobring a longitudinal axis of the slit aperture into coincidence with aflattest meridian of astigmatism whereby gradually changing the slitwidth of the slit aperture so that the laser beam that passes throughthe circular aperture and the slit aperture ablates more amount as it iscloser to the flattest meridian and less amount as it is farther fromthe flattest meridian, second control mean for controlling the opticalsystem in a manner that a longitudinal axis of the slit-like laser beamin a rectangular shape, which is limited by the circular aperture, ismade parallel to a steepest meridian of astigmatism whereby graduallychanging an eccentricity amount of the laser beam so that the laser beamthat passes through the circular aperture and the slit aperture ablatesless amount as it is closer to the steepest meridian and more amount asit is farther from the steepest meridian, and arithmetic means fordividing amount of astigmatic correction into halves approximatelyequally on the basis of the inputted data so that an approximately halfof the astigmatic correction is achieved by the first control means andthe residual astigmatic correction is achieved by the second controlmeans. The optical system includes a circular aperture of which openingdiameter is variable, a slit aperture of which slit width is variable, aprojection lens for projecting the apertures onto the cornea, a movingunit for making an area irradiated by the laser beam eccentric withrespect to a center of the optical zone, and a rotator for rotating thelaser beam.

[0014] Further, in another aspect of the present invention, an apparatusfor corneal surgery apparatus to correct a refractive error by ablatingan optical zone os a cornea with a laser beam, the apparatus comprisesinput means utilized for inputting each data necessary for correction,an optical system, first control means for controlling the opticalsystem in a manner that the scan mirror makes the laser beam in therectangular shape scan an area limited by the circular aperture having adiameter larger than that of the optical zone and by the slit apertureof which longitudinal axis is brought into coincidence with a flattestmeridian of astigmatism whereby gradually changing the slit width of theslit aperture after each scan, second control means for controlling theoptical system in a manner that a longitudinal axis of the laser beam inthe rectangular shape, which is limited by the circular aperture havinga diameter larger than that of the optical zone, is made parallel to asteepest meridian of astigmatism whereby gradually changing aneccentricity amount of the laser beam with respect to the steepestmeridian by the scan mirror so that the laser beam ablates more amountas it is farther from the steepest meridian, and arithmetic means fordividing amount of astigmatic correction into halves approximatelyequally on the basis of the inputted data so that an approximately halfof the astigmatic correction is achieved by the first control means andthe residual astigmatic correction is achieved by the second controlmeans. The optical system includes a circular aperture of which openingdiameter is variable, a slit aperture of which slit width is variable, aprojection lens for projecting the apertures onto the cornea, a scanmirror for making the laser beam in a rectangular shape in a manner thatit crosses the opening of the circular aperture or slit aperture, and animage rotator for rotating the laser beam on an optical axis of theoptical system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate embodiments of thepresent invention and, together with the description, serve to explainthe objects, advantages and principles of the invention. In thedrawings:

[0016]FIG. 1 is a view showing a schematic configuration of an apparatusfor corneal surgery according to the present invention;

[0017]FIG. 2 is a view showing an example of distribution of a cornealrefractive power that is used in a simulation of astigmatic correction;

[0018]FIG. 3 is a view showing a refractive power obtained as a resultof a simulation of myopic astigmatic correction performed on the corneashown in FIG. 2; and

[0019]FIG. 4 is a view showing a refractive power obtained as a resultof a simulation of hyperopic astigmatic correction performed on thecornea shown in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] A detailed description of one preferred embodiment of anapparatus for corneal surgery embodying the present invention will nowbe given referring to the accompanying drawings.

[0021]FIG. 1 is a view showing a schematic configuration of theapparatus for corneal surgery of the present invention.

[0022] Reference numeral 1 is a laser source and used in this embodimentis an excimer laser whose wavelength is 193 nm. A laser beam which isemitted from the laser source 1 is pulsed light of which typical shapeis as follows. The intensity distribution of the beam shows almostuniform distribution in a horizontal direction, and Gaussiandistribution in a vertical direction. Further, the laser beam has a slimrectangle cross-sectional shape when taken along an orthogonal planerelatives to an optical axis L. This is shaped into an intended shape bybeam shaping means such as an expander lens as necessary.

[0023] The laser beam emitted from the laser source 1 is deflectedupward by a plane mirror 2, then into a horizontal direction by a planemirror 3. The plane mirror 3 is movable by a mirror driving device 4 inthe direction of the arrow shown in FIG. 1 so as to move the laser beamin parallel to the direction of the Gaussian distribution for effectinguniform ablation of an object.

[0024]5 is an image rotator which is rotated on the optical axis L by animage rotator driving device 6 so as to rotate the laser beam on theoptical axis L. 7 is a variable circular aperture which limits anablation region. The opening region of the circular aperture 7 ischanged by a circular aperture driving device 8. 9 is a variable slitaperture which limits the ablation region to a slit shape and can changethe opening width by a slit aperture driving device 10. Further, theslit aperture 9 is rotated on the optical axis L by a slit aperturerotate driving device 11 so that the direction of the slit opening maybe changed. The slit aperture 9 is used when correcting myopicastigmatism and the like.

[0025]12 is a projection lens to project the circular aperture 7 and theslit aperture 9 on a cornea 15 of a patient's eye. The region, which isrestricted by the circular aperture 7 and the slit aperture 9 forms animage on the cornea 15.

[0026]13 is a dichroic mirror has a characteristic of reflecting theexcimer laser beam of 193 nm and transmitting visible light. Afterpassing through the projection lens 12, the laser beam is reflected bythe dichroic mirror 13 and directed onto the cornea 15. 14 is anobservation optical system comprising a binocular operation microscope(a commercially available one may be used) is positioned above thedichroic mirror 13. The patient's eye is preliminary positioned at apredetermined position upon an operation and kept the positioning stateby watching a not illustrated fixation light.

[0027]21 is a data input device for inputting required data to correctthe patient's eye. 22 is an arithmetic device for obtaining a requiredablation amount on the basis of the inputted data.

[0028]20 is a control device for controlling the laser source 1 as wellas each of the driving devices 4, 6, 8, 10 and 11 to correct therequired ablation amount obtained by the arithmetic device 22.

[0029] Next, descriptions are given briefly and separately to ablationfor the myopic correction, hyperopic correction, myopic astigmaticcorrection, hyperopic astigmatic correction by controlling drive of theplane mirror 3, the image rotator 5, the circular aperture 7 and thesilt aperture 9.

[0030] Myopic Correction

[0031] In the case of performing myopic correction, the ablation regionis limited with the circular aperture 7 and the laser beam is moved in adirection of the Gaussian distribution by moving the plane mirror 3 stepby step. After the laser beam sweeps along one plane (one scanning), byrotating the image rotator 5, the moving direction of the laser beam ischanged (for example, into three directions at 120° intervalstherebetween), thereby to ablate the exposed area limited with thecircular aperture 7 approximately uniformly. By repeating this procedurewith a different opening size of the circular aperture 7, which ischanged in a stepwise fashion, the cornea is ablated deeper at thecenter and less at the periphery (see U.S. Pat. No. 5,637,109 fordetail).

[0032] Hyperopic Correction

[0033] In the case of performing hyperopic correction, the circularaperture 7 limits the ablation region with the opening region thereofbeing fixed. The plane mirror 3 is made shifted with respect to theoptical axis L thereby to shift the path of the laser beam, and theimage rotator 5 is made rotate so as to ablate the cornea in an annularshape. As the plane mirror 3 moves stepwise and therefore the shiftamount of the laser beam from the optical axis L becomes greater, thenumber of the irradiation pulses (the irradiation time) is increased. Asthe result, the cornea is ablated at a relatively shallow depth at thecenter and at a deeper depth at the periphery for effecting hyperopiccorrection. The thus obtained refractive power is controlled by changingthe total number of the irradiation pulses while keeping the sameproportion among the number of the irradiation pulses at each pointwhere the laser beam is shifted from the optical axis L due to themovement of the plane mirror 3 (see U.S. Pat. No. 5,800,424 for thedetail).

[0034] Hyperopic Astigmatic Correction

[0035] In the case of performing hyperopic astigmatic correction, thecircular aperture 7 limits the ablation region with the opening regionthereof being fixed, and the plane mirror 3 is moved stepwise so as tomove the laser beam in a direction of the Gaussian distribution. At thattime, the image rotator 5 is rotated to adjust the moving direction ofthe laser beam shaped into a slit-like (rectangular) shape so that thelaser beam moves in a direction of the flattest meridian of astigmatism(the image rotator 5 is rotated to adjust so that a longitudinal axis ofthe slit-like laser beam in a rectangular shape is made parallel to thesteepest meridian of astigmatism). The laser beam is moved stepwise withthe number of the irradiation pulses at each point increased in asuitable ratio as the laser beam shifted to the periphery. As theresult, the cornea is ablated at a relatively shallow depth at thecenter and at a deeper depth at the periphery (see U.S. Pat. No.5,800,424 for the detail).

[0036] Myopic Astigmatic Correction

[0037] In the case of performing myopic astigmatic correction, theopening region of the circular aperture 7 is fixed, and the openingwidth of the slit aperture 9 is changed. In addition, the slit aperture9 is adjusted its direction of the slit by the slit aperture rotatedriving device 11 so that the opening width changes in a direction ofthe steepest meridian of astigmatism (the slit aperture 9 is adjustedits direction of the slit so as to bring a longitudinal axis of the slitaperture 9 into coincidence with the flattest meridian of astigmatism).The laser beam irradiation is carried out in the same manner as themyopic correction. That is, the laser beam is made move in a directionof the Gaussian distribution by moving the plane mirror 3 stepwise, andafter each scan of the laser beam, the moving direction of the laserbeam is changed by the rotation of the image rotator 5. As the result,the range limited by the circular aperture 7 and the slit aperture 9 isablated approximately uniformly. By repeating this procedure with adifferent opening width of the slit aperture 9, which is changedstepwise, ablation is done in a manner to flatten the cornea in adirection of the steepest meridian.

[0038] Further, it is also possible to increase the number of theirradiation pulses, as in the case of hyperopic astigmatic correction,at each point where the laser beam moves stepwise but in a reverse way:the number of the irradiation pulses is increased as the laser beamirradiates a closer point to the center (see U.S. Pat. No. 5,800,424 forthe detail).

[0039] Description is now given to ablation for astigmatic correctionaccording to the present invention.

[0040] There have been cases where data indicating partialovercorrection is found upon checking a corneal topography map afterperforming myopic astigmatic correction alone by using a slit apertureand the like. Therefore, simple simulations were carried out.

[0041] As shown in FIG. 2, a cornea has a preoperative refractive poweris 45 D in a 90° direction and 40 D in a 180° direction. A simplesimulation was carried out on this cornea to see a postoperativerefractive power along each meridian induced by ablation on a zone of 5mm in a manner to remove a cylindrically shaped portion having anastigmatism power (cylindrical power) C=−5.0 D, an angle of astigmaticaxis A=180° thereby to reshape the cornea into a spherical shape. It isclear from the result, as shown in FIG. 3, that there are portions beingovercorrected in a certain pattern. This is considered to be the factor,after performing myopic astigmatic correction which ablates deeper atthe center and less at the periphery in the steepest meridian direction,that causes hyperopic shift of the spherical component and undercorrection of the astigmatic component.

[0042] In addition, another simulation was performed on the corneahaving the same refractive power as stated above to see a postoperativerefractive power in a direction of each meridian after performinghyperopic astigmatic correction to ablate a cylindrical shaped portionhaving an astigmatism power C=+5.0 D, an angle of astigmatic axis A=90°thereby to reshape the cornea into a spherical shape. It is clear fromthe result as shown in FIG. 4, there are portions of undercorrection,contrary to the case of myopic astigmatic correction, in a similarpattern.

[0043] Here, in the present invention, a step of hyperopic astigmaticcorrection and a step of myopic astigmatic correction are performed incombination to correct astigmatism, and upon combining the two steps,the amount of the astigmatic correction are divided into halves (equalpower). (It does not have to be divide strictly equally in practice.)The combination eliminates adverse effects of each correction allowingto reshape a cornea into a perfectly spherical shape. Thereafter, byperforming a step of spherical correction (the above-mentioned myopic orhyperopic correction) for the spherical equivalent of correctiverefractive power, the residual correction may be effected.

[0044] That is, letting S denote spherical power of the correctedrefractive power, and C denote the astigmatism power (taken with a minussign), and A denote an angle of an astigmatic axis, then an amount to becorrected in each step is calculated as follows:

[0045] (1) First step (hyperopic astigmatic correction),

[0046] S₁=0, C₁=−C/2, A₁=A+90 (A=0° ˜90°) or A₁=A−90 (A=91° ˜180°);

[0047] (2) Second step (myopic astigmatic correction),

[0048] S₂=0, C₂=C/2, A₂=A; and

[0049] (3) Third step (spherical correction),

[0050] S₃=S+C/2.

[0051] The above arithmetic operations are performed by the arithmeticdevice 22 on the data, such as a preoperative corneal shape of thepatient's eye and a corrected refractive power to be obtained, inputtedwith the use of the data input device 21.

[0052] For example, in the case where the correction amount of therefractive power is S=−1.0 D, C=−5.0 D and A=180° the refractive powerin each principal meridian direction is −6.0D in a 90° direction and−1.0 D in a 180° direction. Here, an amount to be corrected in each stepis calculated as follows.

[0053] (1) First step (hyperopic astigmatic correction)

[0054] S₁=0 D, C₁=+2.5 D and A₁=90°:

[0055] as the result of the hyperopic astigmatic correction, the thusobtained refractive power will be −6.0 D in a 90° direction and −3.5 Din a 180° direction.

[0056] (2) Second step (myopic astigmatic correction)

[0057] S₂=0 D, C₂=−2.5 D and A₂=180°:

[0058] as the result of the myopic astigmatic correction, the thusobtained refractive power will be −3.5 D in a 90° direction and −3.5 Din a 180° direction.

[0059] (3) Third step (spherical correction)

[0060] S₃ =−3.5 D, which is a spherical equivalent of the correctionamount of the refractive power.

[0061] By performing the three steps, the thus obtained refractive powerwill be 0 D in both principal meridian directions, which means that allthe correction amount of the refractive power is done all the way. Inaddition, since the astigmatic correction is also effected by hyperopicastigmatic correction and the myopic astigmatic correction half-andhalf, the adverse effects of each correction are canceled, and thereforethe both refractive power in each meridian between each principalmeridian will be approximately 0 D.

[0062] Here, it should be noted that the order of performing each stepis not limited as described above. Yet, it is preferable to firstperform hyperopic correction which requires more precise alignment ofthe laser irradiation optical system with the patient's eye, and toperform myopic astigmatic correction proceeding to the sphericalcorrection.

[0063] Hereinafter, another example will be described. In the case wherethe correction amount of the refractive power is S=+4.0 D, and C=−8.0 Dand A=90°, an amount to be corrected in each step is calculated asfollows:

[0064] (1) First step (hyperopic astigmatic correction) S₁=0 D, C₁ =+4.0D and A₁=180°;

[0065] (2) Second step (myopic astigmatic correction) S₂=0 D, C₂ =−4.0 Dand A₂=90°; and

[0066] (3) Third step (spherical correction) S₃=0 D, which does not needto be performed.

[0067] In the above examples, it is proffered to add a fourth step,after the corrective laser irradiation, of performing approximatelyuniform ablation for smoothing the laser irradiated surfaces. Thisablation is achieved by a laser irradiation in a manner of PTK(phototherapeutic keratectomy) which is therapeutic superficialkeratectomy. This ablation for smoothing is performed in the followingmanner.

[0068] First, upon limiting the ablation region, the opening region ofthe circular aperture 7 is set to a larger size than the region to becorrected. The laser beam irradiation is carried out in the same manneras the myopic correction: that is, the plane mirror 3 is moved stepwiseso as to move the laser beam in a direction of the Gaussiandistribution, and after each scanning, the image rotator 5 rotates so asto change the moving direction of the laser beam irradiation (into threedirections at 120° intervals therebetween). As the result, the regionlimited by the circular aperture 7 is ablated approximately uniformly.

[0069] For example, in the case of performing corrective ablation on azone of 5.5-6.5 mm, ablation for smoothing is performed on a region of9.0 mm and at a depth of 30μ. At this time, the control device 20controls the laser irradiation of the laser source 1 so as to make therepetition frequency of the laser pulse 10 Hz.

[0070] The smoothing ablation, in practice, is performed withinstillation of an ophthalmic solution having low viscosity (sodiumhyaluronic acid of low molecular weight). Since the ophthalmic solutionpools in concaves in the corneal surface leaving convexes above thesolution level, the laser beam irradiation onto the cornea in such astate ablates only the convexes as the concaves are masked by thesolution. Consequently, the unevenness in the surface is reduced to makeit smoother. The repetition-rate of the laser pulse is 10 Hz, which isquite slow, to allow time for the ophthalmic solution to be back in theconvexes after it is scatted by the preceding pulse of the laserirradiation.

[0071] Still further, another example is described hereinafter. Forexample, in the case where the correction amount is S=−6.0 D, C=−4.0 Dand A=180°, an amount to be corrected in each step is as follows:

[0072] (1) First step (hyperopic astigmatic correction) S₁=0 D, C₁=+2.0D and A₁=90°

[0073] (2) Second step (myopic astigmatic correction) S₂=0 D, C₂=−2.0 Dand A₂=180°; and

[0074] (3) Third step (spherical correction) S₃=−8.0 D.

[0075] In this case, the ablation amount of the myopic correction isrelatively large. That is, if the myopic astigmatism corrective ablationand the myopic corrective ablation are performed on a zone of the samesize, the ablation depth at the center portion tends to be too deep.

[0076] To avoid the above situation, a multi-zone is applied to thesecond step so as to differentiate the size stepwise. To differentiatethe size, the optical zone is denoted by OZ (mm) while the transitionzone is denoted by TZ (mm), and then the size of the transition zone isobtained in the expression, TZ=OZ+2.0 mm. Then, the diameter of theirradiation is enlarged by 0.5 mm at every astigmatic correction amountof −0.5 D. The myopic correction of the spherical component is performedon the same size at every −1.0 D.

[0077] In the case of the above example, the spherical correction (themyopic correction) for S₃=−8.0 D, and the myopic astigmatic correctionfor C₂=−2.0 need to be effected. So, correction for S₂=−1.0 D, C₂=−0.5 Dand A₂=180° is performed on the following four sizes (the sphericalcorrection for S₂=−1.0 D and the myopic astigmatic correction forC₂=−0.5 D and A₂=180° are performed separately):

[0078] (1) OZ 6.5/TZ 8.5 mm;

[0079] (2) OZ 6.0/TZ 8.0 mm;

[0080] (3) OZ 5.5/TZ 7.5 mm; and

[0081] (4) OZ 5.0/TZ 7.0 mm.

[0082] Thereafter, the rest of the spherical correction for S₃=−4.0 D isperformed as the third step. As described above, by differentiating thesize of the zone stepwise upon performing the myopic astigmaticcorrection (and simultaneously, the myopic correction), the ablationdepth at the center is made generally shallow.

[0083] Here, in the above example, the first step of the hyperopiccorrection is performed under the condition where OZ 5.5/TZ 9.0 mm, andthe third step of the spherical correction is performed under thecondition where OZ 5.0/TZ 7.0 mm. Further, also in this case, thesmoothing ablation should be performed as the forth step.

[0084] Data for differentiating the size with the use of multi zone asderived above is calculated by the arithmetic device 22 at the input ofthe data necessary for correction of the patient's eye from the datainput device 21. The control device 20 contains a program for performingthe ablation for hyperopic astigmatic correction, myopic astigmaticcorrection, spherical correction and for smoothing in sequence. Inaddition, the control device 20 controls each device based on thecontrol data for laser irradiation obtained from the arithmetic resultand the inputted data upon carrying out the laser irradiation. Thegradual change of the optical zone and the transition zone is achievedby controlling the opening region of the circular aperture 7. For thedetails of the way to form the transition zone, see U.S. Pat. No.5,445,633.

[0085] As described above, the present invention eliminates the need toobtain the hyper shift rate upon astigmatic correction as an empiricalvalue. In addition, the present invention enables astigmatic correctionwith reducing the effect of change that the spherical componentundergoes without any limitation of a specified correction range.

[0086] Further, the program for differentiating the size according tothe correction amount upon the myopic astigmatic correction and also theprogram for smoothing ablation after the laser beam irradiation foreffecting the correction is added to the sequence of the procedure. Thiseliminates the need to input data for each correction and thereforeinput errors are prevented allowing to perform surgery efficiently.

[0087] In the above embodiments, the so-called slit scanning method isapplied as the laser irradiation method. Yet, the following methods arealso applicable as an ablation method for correcting astigmatism byablating the cornea in a step-like shape symmetrically with respect tothe astigmatic axis. The methods are the spot scanning method, theone-shot irradiation method, a modified method of the above threemethods. For example, a modified method of the slit scanning method orthe one-shot irradiation method is disclosed in U.S. Pat. No. 5,713,892.In this method, a laser beam irradiates a corneal surface via variableblades and the width of a slot and the diameter of an iris are varied asthe laser is pulsed to produce toric ablation of the corneal surface.Another example is disclosed in EP 628298 which is a modification of theslit scanning method for hyperopic correction. In this method, the imageof a variable aperture such as a variable width slit and a variablediameter iris diaphragm, is scanned in a preselected pattern to performablative sculpting of a corneal surface. The scanning is performed witha movable image offset displacement mechanism having a projection lenswhich is capable of effecting radial displacement and angularly rotationof the profiled laser beam exiting from the variable aperture.

[0088] The foregoing description of the preferred embodiments of theinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and modifications andvariations are possible in the light of the above teachings or may beacquired from practice of the invention. The embodiments chosen anddescribed in order to explain the principles of the invention and itspractical application to enable one skilled in the art to utilize theinvention in various embodiments and with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the claims appended hereto, and theirequivalents.

What is claimed is:
 1. An apparatus for corneal surgery to correct a refractive error by ablating corneal tissue with a laser beam emitted from a laser source and delivered onto a cornea of a patient's eye with a light delivering optical system, the apparatus comprising: irradiation area limiting means for limiting an irradiation area of the laser beam and for varying the irradiation area; first control means for controlling the irradiation area limiting means so as to reduce an ablation amount as the laser beam irradiates further away from a flattest meridian of astigmatism whereby effecting astigmatic correction; second control means for controlling the irradiation area limiting means so as to increase an ablation amount as the laser beam irradiates further away from a steepest meridian of astigmatism whereby effecting astigmatic correction; and arithmetic means for dividing a refractive power required for astigmatic correction into halves approximately equally so that an approximately half of the astigmatic correction is achieved by the first control means and the residual astigmatic correction is achieved by the second control means.
 2. The apparatus for corneal surgery according to claim 1 , wherein the irradiation area limiting means includes: a circular aperture of which opening diameter is variable; and a slit aperture of which slit width is variable.
 3. The apparatus for corneal surgery according to claim 2 , wherein the irradiation area limiting means further includes: beam rotating means for shifting a position of a boundary of the slit-shaped laser beam and rotating the laser beam being displaced from an axis of the patient's eye on the axis of the patient's eye.
 4. The apparatus for corneal surgery according to claim 1 , wherein the irradiation area limiting means includes: a circular aperture of which opening diameter is variable; a slit aperture of which slit width is variable; boundary shifting means for shifting a position of a boundary of the slit-shaped laser beam; and beam rotating means for rotating the laser beam which is displaced from an axis of the patient's eye on the axis of the patient's eye,-and the first control means approximately uniformly ablates an area limited by the slit aperture with the laser beam as well as changes the area limited by the slit aperture gradually.
 5. The apparatus for corneal surgery according to claim 1 , wherein the irradiation area limiting means includes: a circular aperture of which opening diameter is variable; a slit aperture of which slit width is variable; boundary shifting means for shifting a position of a boundary of the slit-shaped laser beam; and beam rotating means for rotating the laser beam which is displaced from an axis of the patient's eye on the axis of the patient's eye, and the second control means controls the boundary position of the laser beam shifted by the boundary shifting means and rotation of the beam rotating means.
 6. The apparatus for corneal surgery according to claim 5 , wherein the boundary shifting means moves a mirror in parallel whereby moving the slit-shaped laser beam in a direction of its width, and the second control means repeats a process of rotating the slit-shaped laser beam limited by the circular aperture on the axis of the patient's eye by the beam rotating means with the laser beam being displaced to a different degree.
 7. The apparatus for corneal surgery according to claim 5 , wherein the boundary shifting means includes: a projection lens for projecting the slit aperture being displaced from the axis of the patient's eye; and said slit aperture of which slit width is variable, and the second control means repeats a process of rotating the slit-shaped laser beam limited by the circular aperture on the axis of the patient's eye by the beam rotating means with the laser beam being displaced to a different degree.
 8. An apparatus for corneal surgery to correct a refractive error by ablating an optical zone of a cornea with a laser beam, the apparatus comprising: input means utilized for inputting each data necessary for correction; an optical system that includes a circular aperture of which opening diameter is variable, a slit aperture of which slit width is variable, a projection lens for projecting the apertures onto the cornea, a moving unit for making an area irradiated by the laser beam eccentric with respect to a center of the optical zone, and a rotator for rotating the laser beam; first control means for controlling the optical system so as to bring a longitudinal axis of the slit aperture into coincidence with a flattest meridian of astigmatism whereby gradually changing the slit width of the slit aperture so that the laser beam that passes through the circular aperture and the slit aperture ablates more amount as it is closer to the flattest meridian and less amount as it is farther from the flattest meridian; second control mean for controlling the optical system in a manner that a longitudinal axis of the slit-like laser beam in a rectangular shape, which is limited by the circular aperture, is made parallel to a steepest meridian of astigmatism whereby gradually changing an eccentricity amount of the laser beam so that the laser beam that passes through the circular aperture and the slit aperture ablates less amount as it is closer to the steepest meridian and more amount as it is farther from the steepest meridian; and arithmetic means for dividing amount of astigmatic correction into halves approximately equally on the basis of the inputted data so that an approximately half of the astigmatic correction is achieved by the first control means and the residual astigmatic correction is achieved by the second control means.
 9. An apparatus for corneal surgery apparatus to correct a refractive error by ablating an optical zone of a cornea with a laser beam, the apparatus comprising: input means utilized for inputting each data necessary for correction; an optical system that includes a circular aperture of which opening diameter is variable, a slit aperture of which slit width is variable, a projection lens for projecting the apertures onto the cornea, a scan mirror for making the laser beam in a rectangular shape in a manner that it crosses the opening of the circular aperture or slit aperture, and an image rotator for rotating the laser beam on an optical axis of the optical system; first control means for controlling the optical system in a manner that the scan mirror makes the laser beam in the rectangular shape scan an area limited by the circular aperture having a diameter larger than that of the optical zone and by the slit aperture of which longitudinal axis is brought into coincidence with a flattest meridian of astigmatism whereby gradually changing the slit width of the slit aperture after each scan; second control means for controlling the optical system in a manner that a longitudinal axis of the laser beam in the rectangular shape, which is limited by the circular aperture having a diameter larger than that of the optical zone, is made parallel to a steepest meridian of astigmatism whereby gradually changing an eccentricity amount of the laser beam with respect to the steepest meridian by the scan mirror so that the laser beam ablates more amount as it is farther from the steepest meridian; and arithmetic means for dividing amount of astigmatic correction into halves approximately equally on the basis of the inputted data so that an approximately half of the astigmatic correction is achieved by the first control means and the residual astigmatic correction is achieved by the second control means. 